The first living organisms originated among organic molecules and in oxygen free atmosphere. They presumably obtained energy by fermentation of some of these organic molecules. They were non-green and anaerobes (capable of respiring in absence of oxygen). They depended on the existing organic molecules for their nutrition, hence they were heterotrophs. Then came autotrophs when the supply of existing organic molecules were exhausted. These organisms were capable of producing their own organic molecules by chemosynthesis. They synthesized food from inorganic molecules. Photoautotrophs were also anaerobic at first but later on oxygen releasing photosynthetic organism developed.